Biological molecules, such as monoclonal antibodies (MAbs) or other proteins/polypeptides, as well as small chemical compounds directed against various receptors and other antigens on the surface of tumor cells are known to be suitable for tumor therapy for more than twenty years. With respect to the antibody approach, most of these MAbs are chimerized or humanized to improve tolerability with the human immune system. Mabs or above-mentioned chemical entities specifically bind to their target structures on tumor cells and in most cases also on normal tissues and can cause different effects that dependent on their epitope specificity and/or functional characteristics of the particular antigen. MAbs to orphan receptors or other non-functional cell surface molecules as well as MAbs against structures outside the ligand-binding site of functionally active receptors (e.g. growth factor receptors with kinase activity) would be expected to induce primarily immune effector functions against the target cell (antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC)). Additionally, depending on the properties of antigen and MAb, binding of the antibody can result in cross-linking of the receptors. Consequent internalization of the receptor-antibody complexes may result in a prolonged down-modulation of the receptor density on the cell surface.
MAbs which bind to an epitope within the ligand-binding site or in its direct neighborhood compete for binding of natural ligands to their receptor and thus reduce or completely inhibit ligand binding and can displace already bound ligands from their receptors. This receptor blockade inhibits ligand-dependent receptor activation and downstream signaling. For example, blockade of ErbB receptors, such as the epidermal growth factor receptor (EGFR), by monoclonal antibodies results in various cellular effects including inhibition of DNA synthesis and proliferation, induction of cell cycle arrest and apoptosis as well as antimetastatic and antiangiogenetic effects.
ErbB receptors are typical receptor tyrosine kinases that were implicated in cancer in the 1980s. Tyrosine kinases are a class of enzymes that catalyze the transfer of the terminal phosphate of adenosine triphosphate to tyrosine residues in protein substrates. Tyrosine kinases are believed, by way of substrate phosphorylation, to play critical roles in signal transduction for a number of cell functions. Though the exact mechanisms of signal transduction is still unclear, tyrosine kinases have been shown to be important contributing factors in cell proliferation, carcinogenesis and cell differentiation. Tyrosine kinases can be categorized as receptor type or non-receptor type. Both receptor-type and non-receptor type tyrosine kinases are implicated in cellular signaling pathways leading to numerous pathogenic conditions, including cancer, psoriasis and hyperimmune responses. Many tyrosine kinases are involved in cell growth as well as in angiogenesis. The non-receptor type of tyrosine kinases is also comprised of numerous subfamilies, including Src, Frk, Btk, Csk, Abl, Zap70, Fes/Fps, Fak, Jak, Ack, and LIMK. Each of these subfamilies is further sub-divided into varying receptors. For example, the Src subfamily is one of the largest and includes Src, Yes, Fyn, Lyn, Lck, Blk, Hck, Fgr, and Yrk. The Src subfamily of enzymes has been linked to oncogenesis. For a more detailed discussion of the non-receptor type of tyrosine kinases, see Bolen Oncogene, 8:2025–2031 (1993).
Receptor type tyrosine kinases have an extracellular, a transmembrane, and an intracellular portion, while non-receptor type tyrosine kinases are wholly intracellular. Receptor-linked tyrosine kinases are transmembrane proteins that contain an extracellular ligand binding domain, a transmembrane sequence, and a cytoplasmic tyrosine kinase domain. The receptor-type tyrosine kinases are comprised of a large number of transmembrane receptors with diverse biological activity.
Different subfamilies of receptor-type tyrosine kinases have been identified. Implicated tyrosine kinases include fibroblast growth factor (FGF) receptors, epidermal growth factor (EGF) receptors of the ErbB major class family, and platelet-derived growth factor (PDGF) receptors. Also implicated are nerve growth Factor (NGF) receptors, brain-derived neurotrophic Factor (BDNF) receptors, and neurotrophin-3 (NT-3) receptors, and neurotrophin-4 (NT-4) receptors.
One receptor type tyrosine kinase subfamily, designated as HER or ErbB subfamily, is comprised of EGFR (ErbB1), HER2 (ErbB2 or p185neu), HER3 (ErbB3), and HER4(ErbB4 or tyro2). Ligands of this subfamily of receptors include epithelial growth factor (EGF), TGF-a, amphiregulin, HB-EGF, betacellulin, heregulin and neuregulins. The PDGF subfamily includes the FLK family which is comprised of the kinase insert domain receptor (KDR).
EGFR, encoded by the erbB1 gene, has been causally implicated in human malignancy. In particular, increased expression of EGFR has been observed in breast, bladder, lung, head, neck and stomach cancer as well as glioblastomas. Increased EGFR receptor expression is often associated with increased production of the EGFR ligand, transforming growth factor alpha (TGF-a), by the same tumor cells resulting in receptor activation by an autocrine stimulatory pathway (Baselga and Mendelsohn, Pharmac. Ther. 64:127–154 (1994)).
The EGF receptor is a transmembrane glycoprotein which has a molecular weight of 170.000, and is found on many epithelial cell types. It is activated by at least three ligands, EGF, TGF-α (transforming growth factor alpha) and amphiregulin. Both epidermal growth factor (EGF) and transforming growth factor-alpha TGF-a) have been demonstrated to bind to EGF receptor and to lead to cellular proliferation and tumor growth. These growth factors do not bind to HER2 (Ulrich and Schlesinger, 1990, Cell 61, 203). In contrary to several families of growth factors, which induce receptor dimerization by virtue of their dimeric nature (e.g. PDGF) monomeric growth factors, such as EGF, contain two binding sites for their receptors and, therefore, can cross-link two neighboring EGF receptors (Lemmon et al., 1997, EMBO J. 16, 281). Receptor dimerization is essential for stimulating of the intrinsic catalytic activity and for the self-phosphorylation of growth factor receptors on tyrosine residues. The latter serve as docking sites for various adaptor proteins or enzymes, which simultaneously initiate many signaling cascades. In higher eukaryotes, the simple linear pathway has evolved into a richly interactive, multi-layered network in which combinatorial expression and activation of components permits context-specific biological responses throughout development. The ErbB network might integrate not only its own inputs but also heterologous signals, including hormones, lymphokines, neurotransmitters and stress inducers.
It should be remarked that receptor protein tyrosine kinases (PTKs) are able to undergo both homo- and heterodimerization, wherein homodimeric receptor combinations are less mitogenic and transforming (no or weak initiation of signaling) than the corresponding heterodimeric combinations. Heterodimers containing ErbB2 are the most potent complexes (see review articles by Yarden and Sliwkowski, 2001, Nature Reviews, Molecular cell Biology, volume 2, 127–137; Tzahar and Yarden, 1998, BBA 1377, M25–M37).
It has been demonstrated that anti-EGF receptor antibodies while blocking EGF and TGF-a binding to the receptor appear to inhibit tumor cell proliferation. In view of these findings, a number of murine and rat monoclonal antibodies against EGF receptor have been developed and tested for their ability inhibit the growth of tumor cells in vitro and in vivo (Modjtahedi and Dean, 1994, J. Oncology 4, 277). Humanized monoclonal antibody 425 (h MAb 425, U.S. Pat. No. 5,558,864; EP 0531 472) and chimeric monoclonal antibody 225 (c MAb 225, U.S. Pat. No. 4,943,533 and EP 0359 282), both directed to the EGF receptor, have shown their efficacy in clinical trials. The C225 antibody (Cetuximab) was demonstrated to inhibit EGF-mediated tumor cell growth in vitro and inhibit human tumor formation in vivo in nude mice. The antibody, moreover, appeared to act, above all, in synergy with certain chemotherapeutic agents (i.e., doxorubicin, adriamycin, taxol, and cisplatin) to eradicate human tumors in vivo in xenograft mouse models. Ye et al. (1999, Oncogene 18, 731) have reported that human ovarian cancer cells can be treated successfully with a combination of both chimeric MAb 225 and humanized MAb 4D5 which is directed to the HER2 receptor.
The second member of the ErbB family, HER2 (ErbB2 or p185neu), was originally identified as the product of the transforming gene from neuroblastomas of chemically treated rats. The activated form of the neu proto-oncogene results from a point mutation (valine to glutamic acid) in the transmembrane region of the encoded protein. Amplification of the human homologue of neu is observed in breast and ovarian cancers and correlates with a poor prognosis (Slamon et al., Science, 235: 177–182 (1987); Slamon et al., Science, 244:707–712 (1989); U.S. Pat. No. 4,968,603). ErbB2 (HER2) has a molecular weight of about 185.000, with considerable homology to the EGF receptor (HER1), although a specific ligand for HER2 has not yet been clearly identified so far. The antibody 4D5 directed to the HER2 receptor, was further found to sensitize ErbB2-overexpressing breast tumor cell lines to the cytotoxic effects of TNFα (U.S. Pat. No. 5,677,171). A recombinant humanized version of the murine anti-ErbB2 antibody 4D5 (huMAb4D5-8, rhuMAb HER2 or HERCEPTIN® ; U.S. Pat. No. 5,821,337) is clinically active in patients with ErbB2-overexpressing metastatic breast cancers that have received extensive prior anti-cancer therapy (Baselga et al., J. Clin. Oncol. 14:737–744 (1996)); HERCEPTIN® received marketing approval in 1998 for the treatment of patients with metastatic breast cancer whose tumors overexpress the ErbB2 protein.
Besides anti-ErbB antibodies there are numerous small chemical molecules which are known to be potent inhibitors of ErbB receptor molecules blocking the binding site of the natural ligands (see detailed description), or blocking the tyrosine residues of the binding site of the receptor kinase, thus preventing phosphorylation and further cascade signaling. One representative showing high efficacy in clinical trials is Iressa™ (ZD-1839) which can be applied for NSCLC indication (non-small cell lung cancer).
Although there are already some promising drugs and methods of treatment tumors under development and in the market, there is a continuous need for further agents and pharmaceutical compositions and combinations with improved properties and enhanced efficacy.